Unbutton the shirt and spread it out on its back. Place one end of the measuring tape at left end of the collar band. Pull the collar tight so that any curve to the collar band becomes straight. Measure straight across the collar band to right end, thus measure collar band end to end. In image, it is 15.3 inches.
Spread the shirt on its back and make sure that there are no wrinkles or folds underneath. Measure from bottom of the armpit seam, straight across, on the same point on other side. Thus measure it armpit to armpit. In image, it is 21.3 inches.
Mid-section / tummy:
Spread the shirt on its back. Find the narrowest part of the body. It is between 5th and 6th button starting from the top collar button. This is probably about halfway between the armpits and the bottom of the shirt. Measure it across the width of the shirt. In image, it is 20.5 inches.
Measure straight across bottom of the shirt at side seams. Do not stretch the shirt apart too much when taking this measurement. In image, it is 21.1 inches.
Button up the shirt and spread the shirt out on its front. Measure from the point where shoulder meets the sleeve, straight across, on the same point on other side. Thus measure it across the seams of the shoulder, edge to edge. In image, it is 17.2 inches.
Spread the shirt on its front. Measure from bottom edge of the middle of the collar to bottom of the shirt. Do not stretch the shirt, but be sure it is lying flat and fully extended, in image, it is 29.9 inches.
Start measuring from Top of the sleeve at shoulder seam and measure straight across sleeve to the edge of the cuff. in image, it is 25.2 inches.
From top of shoulder seam to the armpit. In image, it is 8.5 inches.
Spread the cuff out flat and measure Cuff end to end. In image, it is 10 inches.